The physiological strain index does not reliably identify individuals at risk of reaching a thermal tolerance limit

Sarah Davey, Victoria Downie, Katy Griggs, George Havenith

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    5 Citations (Scopus)
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    Purpose: The physiological strain index (PSI) was developed to assess individuals’ heat strain, yet evidence supporting its use to identify individuals at potential risk of reaching a thermal tolerance limit (TTL) is limited. The aim of this study was to assess whether PSI can identify individuals at risk of reaching a TTL. Methods: Fifteen females and 21 males undertook a total of 136 trials, each consisting of two 40–60 minute periods of treadmill walking separated by ~ 15 minutes rest, wearing permeable or impermeable clothing, in a range of climatic conditions. Heart rate (HR), skin temperature (T sk), rectal temperature (T re), temperature sensation (TS) and thermal comfort (TC) were measured throughout. Various forms of the PSI-index were assessed including the original PSI, PSI fixed, adaptive-PSI (aPSI) and a version comprised of a measure of heat storage (PSI HS). Final physiological and PSI values and their rate of change (ROC) over a trial and in the last 10 minutes of a trial were compared between trials completed (C, 101 trials) and those terminated prematurely (TTL, 35 trials). Results: Final PSI original, PSI fixed, aPSI, PSI HS did not differ between TTL and C (p > 0.05). However, differences between TTL and C occurred in final T sk, T re–T sk, TS, TC and ROC in PSI fixed, T re, T sk and HR (p < 0.05). Conclusion: These results suggest the PSI, in the various forms, does not reliably identify individuals at imminent risk of reaching their TTL and its validity as a physiological safety index is therefore questionable. However, a physiological-perceptual strain index may provide a more valid measure.

    Original languageEnglish
    Pages (from-to)1701-1713
    Number of pages13
    JournalEuropean Journal of Applied Physiology
    Issue number6
    Early online date7 Mar 2021
    Publication statusPublished - Jun 2021

    Bibliographical note

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    European Commission for project funding FP7-NMP-2008SME-2, Proj. No. 229042:
    PROSPIE—Protective Responsive Outer Shell for People in Industrial Environments.


    • Physiological strain index
    • Heat stress
    • Heat stress indices
    • Thermal tolerance limit
    • Heat illness
    • Hyperthermia-induced fatigue

    ASJC Scopus subject areas

    • Public Health, Environmental and Occupational Health
    • Physiology (medical)
    • Orthopedics and Sports Medicine


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